1. Field of the Invention
This invention relates to a process for curing epoxy powder coating compositions and to epoxy powder coating compositions utilizing an isocyanate blocked imidazole and imidazoline catalyst.
2. Description of the Prior Art
In recent years there has been a substantial amount of work in the development of powder coating systems for application to a variety of substrates. The use of powder coatings overcomes many of the environmental problems liquid, resinous coating compositions have presented in manufacturing, particularly where a volatile organic solvent is used as the carrier.
Essentially two methods of formulating epoxy powder coating systems have been used. One method is to grind the epoxy powder coating and combine it with various additives, pigments and catalysts in a blender and then apply this physical blend to a substrate and cure at elevated temperature. The second technique has been to melt mix the epoxy resin with various additives, pigments and catalysts and extrude this melt mix into a thin ribbon and cooling to form a solid. This solid is then ground into a finely divided form, e.g. 250 microns or less depending on whether the coating is to be used for decorative or functional purposes. The latter method of formulating epoxy powder coatings has significant advantages in that it permits greater uniformity of dispersion of additives, pigments, and catalyst within the resin itself so that the end coating is not spotty and nonuniform. It also presents the most difficulty in formulation due to the fact that many catalysts are too reactive at the melt mix temperature, and those that are not are too slow at cure temperature or require a high cure temperature.
There are a number of patents which disclose the use of various catalyst systems for epoxy powder coatings and catalyst systems based on imidazoles and imidazoline. Representative of these patents are:
U.S. Pat. No. 4,041,007 discloses a combination of imidazole and substituted imidazoles with clay filler and an aromatic amine as a catalyst system for epoxy resins. The particular combination of clay filler and imidazole retards the catalytic activity of the aromatic amine thereby enhancing the pot life of the epoxy resin.
U.S. Pat. No. 3,896,082 discloses the use of imidazoline derivatives as a hardner for epoxy powder coating systems. The imidazolines are alleged to eliminate long setting times and high cure temperatures. In addition, the catalyst properties improved leveling characteristics in gloss and increased resistance to washing solutions.
U.S. Pat. No. 3,956,237 discloses the use of a latent amine curing agent and an accelerator for epoxy resin compositions. The latent amine curing agents are prepared by reacting alpha-substituted aromatic mono or diisocyanates with an alkyl or phenyl substituted secondary amine or a saturated heterocyclic amine such as pyrrolidine or alkyl substituted pyrrolidine. These accelerators are used in combination with a latent amine such as dicyandiamide. One of the problems with the accelerators disclosed here is that they are not extremely reactive without the dicyandiamide present.
U.S. Pat. No. 3,677,978 discloses that imidazoles can be used to effect a rapid cure of epoxy resins and that the shelf life of the epoxy resins can be extended by including a polyvalent metal salt, e.g. a copper salt.
U.S. Pat. Nos. 3,931,117 and 3,898,978 disclose the use of an epsilon-caprolactam organic polyisocyanate as a catalyst for hydroxyl containing polymers, e.g. polyesters, hydroxycontaining acrylics and epoxy resins.
U.S. Pat. No. 4,041,019 discloses an isocyanate capped imidazole as a delayed-action, heat activated catalyst for effecting cure of polyurethanes. An example of an isocyanate capped imidazole is a toluene diisocyanate capped imidazole. Curing temperatures for the urethanes are approximately 120.degree. C.
U.S. Pat. No. 3,989,673 discloses a catalyst system which is effective for curing epoxy resins at low temperatures. The catalyst system is a substituted imidazole such as 2-ethyl-4-methyl imidazole and piperidine.
U.S. Pat. No. 4,007,299 discloses the formation of a coated substrate having a matt finish from a mixture of a powdered epoxy compound, and a N-containing salt of a carboxylic acid having three or more carboxyl groups and a cyclic amidine by heating the mixture to a temperature of about 175.degree.-220.degree. C. An example of an N-containing salt and a cyclic amidine is the reaction product of imidazoline and trimellitic acid.
U.S. Pat. No. 3,909,480 discloses an epoxy resin composition having excellent curability and storage stability which includes as the catalyst system a complex of a tetraphenyl borate and imidazole.
U.S. Pat. No. 3,538,039 discloses a heat curable epoxy resin composition which is stable at room temperature, but cures in 1 to 5 minutes at 275.degree. to 300.degree. F. The curable composition is an adduct of a polyepoxide and an amine, a polyfunctional anhydride such as trimellitic anhydride and an activator such as an imidazole or substituted imidazole.